A hydraulic accumulator is essentially an energy storage device. Accumulators are widely used in mobile and industrial hydraulics to store energy, dampen pulsations, compensate for thermal expansion, and/or provide auxiliary power. It generally consists of a high pressure vessel in which a non-compressible hydraulic fluid is held under pressure by an external source. These accumulators are based on the principle that gas is compressible and fluid (e.g., oil or other similar liquid) is relatively incompressible. In operation, fluid or oil flows into the accumulator and compresses the gas by reducing its storage volume. Energy is stored in the compressed gas held under pressure. If the fluid is released, it will quickly flow out under the pressure of the expanding gas, thereby dispensing the stored energy.
A bladder accumulator consists of pressure vessel with an internal elastomeric bladder with pressurized nitrogen inside the bladder and hydraulic fluid outside the bladder but contained within the vessel. The accumulator is charged with gas, typically nitrogen, through a valve installed on the top. In a bladder accumulator, the energy is stored by compressing the gas encapsulated within an elastomeric (e.g., rubber) bladder. Energy is released when the hydraulic fluid out of the accumulator's fluid port, thereby decompressing the bladder by allowing it to expand.
The main advantages of a bladder accumulator are fast acting, no hysteresis, not susceptible to contamination, lower cost and consistent behavior under similar conditions. However, bladder accumulators have limitations in applications that require extremely high flow rates, tolerance of temperature extremes, high compression ratios, ability to withstand external forces and/or mounting restrictions. In addition, bladder accumulators typically cannot provide peak power when mounted horizontally or when they are subjected to centrifugal forces perpendicular to their longitudinal direction.
Piston accumulators alleviate many of these issues. A piston accumulator has a piston which slides against the accumulator housing on seals. On one side of the piston is a gas (again typically nitrogen) and on the other side is the hydraulic fluid and connection to the system. A fill port allows pressurization of the nitrogen. One of the advantages of piston accumulators is its ability to provide higher mass flow rate of the hydraulic fluid than bladder accumulators. This means that piston accumulators promise a higher specific power (delivered power per mass of the accumulator) that can be advantageous in mobile applications. Piston accumulators also do not have a bladder that has a finite fatigue life resulting from severe deformation in each cycle, thereby requiring replacement of bladders at regular intervals. In contrast, the seals in the reciprocating piston typically do not require maintenance as frequent as bladder accumulators.
As discussed above, bladder accumulators have limitations when operating at extremely cold or warm temperatures. In contrast, depending on the type of seal used, piston accumulators can have application in a much wider temperature range.
Failure of bladder accumulators is typically sudden and results in leaking of their stored gas into the hydraulic system. In contrast, piston accumulators, because of their small seal surface, generally tend to fail gradually. Thus, even when the piston accumulators begin to fail, the migration of gas from the gas side to the fluid side is slow, leaving a sufficient time for servicing to correct gas leaks into the hydraulic fluid system.
While bladder accumulators generally perform best when mounted vertically with the fluid port at the bottom and gravity assisting in the flow of the fluid, piston accumulators can be mounted in any position. In addition, the performance of bladder accumulators is significantly reduced when subjected to centrifugal forces or Coriolis forces. A piston accumulator is not affected by these forces.
Unfortunately, traditional piston accumulators are made of steel with elaborate machining operation and are generally very heavy. Typically, thick steel cylindrical chambers are used to support the structural load as well as to house the reciprocating piston. Some accumulator manufacturers have attempted to reduce the overall weight of these piston accumulators by substituting steel with structural composite overwrapped over legacy steel chamber designs.
Despite many different hydraulic pressure accumulators that are available, there is a continuing need for a lightweight serviceable hydraulic pressure accumulator.